Amplifier circuits



G. T. FORD AMPLIFIER CIRCUITS June 15, 1937.

Filed NQV. 21, 1955 FIG! . l/ENTOP GA VLON TFORD ATTORNEY Patented June 15, 1937 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

New York, N. Y., a

Incorporated. corporation of New York Application November 21, 1935, Serial No. 50,908

3 Claims.

The present invention relates to the variable control of transmission through a circuit for signaling, indicating, and like purposes, and particularly to the use of thermosensitive material in 5 such a circuit.

While not limited in its broader aspects thereto, the invention is especially concerned with the use of silver sulphide for current control purposes, this material having such a large temperature coefiicient of resistance that resistance changes of the order of ten or one hundred or even more are readily obtained by variations in temperature within ranges that can be conveniently produced and controlled.

An object of the invention is to control the volume or amplitude of waves transmitted through an amplifier circuit under control of the output current by means of a thermosensitive element such as silver sulphide.

A related object is to control the gain of an amplifier circuit in a simple manner independently of any control of amplifier tube characteristics.

Other objects and the various features of the invention will appear as the description proceeds.

Silver sulphide is noteworthy for its large negative temperature coefiicient of resistance, the resistancetemperature relation for this material being given by the equation Where R0 is the resistance at t=0 and t is the temperature on the centigrade scale. This relation holds for temperatures up to about 179 C. at which point the resistance suddenly falls by a factor of about with further increase of temperature of about one degree. A practical notion of the size of the coeflicient for this mate- 40 rial may be had from considering that the resistance of a sample of silver sulphide is substantially divided by Z by a temperature rise of about 14 C. within the temperature range indicated above. The characteristics of the material are such that large factors of resistance change can be obtained in the vicinity of room temperature and higher temperatures without having to go to excessive temperature levels. Resistances composed of this material may be obtained by compressing chemically pure silver sulphide powder at a sufficiently high pressure to form a compact body, for example, 16,000 pounds per square inch or by immersing a silver wire in saturated vapor of boiling sulphur for a sufliciently long time to convert the silver entirely to silver sulphide.

Terminals can be secured to the block or wires so formed by brightening the ends of the material and soldering or tinning leads to the sample. Silver sulphide is unstable when subjected for considerable periods to direct current but has stable and reproducible characteristics when used with alternating current.

The nature of the invention will be more fully comprehended from the following detailed description of specific embodiments as illustrated in the attached drawing forming a part of the specification.

In the drawing:

Fig. l is a schematic circuit diagram of a volume or amplitude control circuit embodying the invention; and I Fig. 2 shows a modified circuit of the same general nature.

Referring to Fig. 1 the line H) may be a telephone line or a circuit used for the transmission of any other desired type of signal that is to be amplified by the amplifier comprising stages I! and I3, and the output circuit or line I I may also be a telephone line or may lead to any suitable type of receiver or reproducer. The signals incoming over line I are transferred through input transformer II to the grid or input circuit of first stage tube I! from which the output waves are transferred to subsequent stage or stages l3, if more than a single stage of amplification is used, and the final output waves are transferred through output transformer I5 to circuit H. The cathodes of the difierent stages are preferably interconnected by wire 24. Space current source 23 is provided for stage l2.

The input or grid circuit of stage l2 comprises high resistance l8 and potentiometer resistance l9 comprised of silver sulphide or other material of suitably high temperature coefiicient of resistance. The waves received from line l0 through input transformer l4 set up a current flow in the circuit including the secondary of input transformer H and resistances l8 and IS in series. The drop of potential appearing across resistance I9 is applied to the grid circuit of the amplifier II.

The silver sulphide resistance I9 is preferably included in a heat chamber 2! in which there is also placed a heating coil 20, the terminals of which are connected by circuit IT to suitable points on resistance l6 connected across the output circuit I I. The arrangement of the elements I9, 20 and 2| may be very simple and compact and, if desired, the inclosure 2| may be omitted entirely. For example, in one case used by applicant the heater element 20 consisted of a coil tube I 2 and any suitable type of No. 46 insulated nichrome wire wound directly on a sample of silver sulphide with no inclosing chamber 2|. If it is desired, however, to control the rate of heat dissipation a heat insulating inclosure 2| may be provided.

In operation, a portion of the output current in resistance I B is led through circuit l1 to the heater 20, which raises the temperature 01' the silver sulphide resistor l9, reducing its resistance and hence tending to decrease the signal voltage applied to the grid circuit 01' the amplifier l2. "With a fairly constant amount of applied wave from the circuit ill a state of equilibrium is reached in which the rate at which heat is lost to the surroundings of resistance I 9 equals the rate at which heat is supplied to resistance H from heater 20. If for any reason the amplitude of the output current tends to increase, more current is fed back through the circuit IT to the heater thus raising the temperature of resistor I9 and lowering its resistance so that a smaller proportion of the input wave is applied to the grid circuit of the tube l2 thus tending to lower the output wave amplitude. If, on the other hand, there is a tendency for the amplitude of the output wave to fall, less heating current is supplied to the resistor l9 so that its temperature falls, its resistance rises, and a larger proportion of the input waves are applied to the id circuit of tube l2.

In this way the circuit as a whole tends automatically to maintain a constant output level or with a difierent adjustment of the circuit the total range of output amplitude is reduced below the range that would be obtained without the control circuits l1, l9 and 20.

It will be noted that the gain control action that has been described takes place without changing the grid bias or any of the other fixed potentials of the amplifier i2. Any suitable type of grid bias (not shown) may be used for the of tube may be used. Electrically, the control is similar to that obtained by a potentiometer with a sliding contact in the grid circuit of the amplifier 12 but the circuit of Fig. 1 avoids the use of a sliding contact or any other mechanically movable part.

By controlling the physical size of the elements l9 and 20 and the heat characteristic'of the heat chamber 2| (if a heat chamber is used), the rapidity of operation of the volume control circuit may be varied within wide limits. For example, it may be rendered insensitive to any except' slow changes in the transmitted wave or it may be made responsive to changes approximating instantaneous amplitude changes of the transmitted waves. In the latter case, the circuit reduces portions of the wave of excessive amplitudes or peaks.

The circuit of Fig. 2 is, in general similar to that of Fig. 1 except that the circuit l1 includes a valve 29 of any suitable type illustrated as a three-electrode gas-filled or trigger tube. This tube is supplied with alternating current voltage from any suitable alternating current source 30 preferably through a transformer 3|. The grid or input circuit of valve 29 includes a biasing battery 32 preferably adjustable.

In the operation of the circuit of Fig. 2 the bias voltage in the tube 29 may be set so that the amplitude control circuit does not come into operation until the amplitude 01' the waves in the output circuit ll exceeds a predetermined minimum value as determined by the adjustment of the bias voltage 32 or slider on resistance IE, or both. When the amplitude oi the waves in the circuit H exceeds the minimum value referred to, the voltage of the grid of tube 29 is driven sufficiently positive to cause the tube to discharge during positive hall waves of the plate potential and send current through the heater thus lowering the resistance of the silver sulphide potentiometer l9 and reducing the amplitude of waves applied to the grid of the tube I 2. This action continues so long as the amplitude of the output wave in circuit H is suificient to drive the grid of the valve 29 positive, or in the positive direction, far enough to permit discharge of the tube. A filter 28 may be provided in circuit I! to permit waves of only certain frequencies to control the discharge of the valve 29. For'example, in the case of a radio receiver, filter 28 may be designed to pass the carrier frequency but not the side-bands so that the gain control is exercised by the carrier wave of the broadcasting station and a change in the strength of the carrier wave is compensated by an appropriate change in the resistances l9 such that the output signal waves and circuit H remain constant.

Various modifications within the spirit and scope of the claims will occur to those skilled in the art in view of the two illustrative forms that have been given.

What is claimed is:

1. A voltage limiting circuit comprising an amplifier having input and output circuits, a silver sulphide resistor in the input circuit, a heater winding associated in heat transfer relation with said resistor, said heater winding being connected to the output circuit of said amplifier whereby output current flows through the heater winding and varies the temperature of said resistor and a valve included in the circuit of the heater winding, biased to permit output current only in excess of a predetermined amplitude to cause current to flow in the heater winding.

2. In a speech transmission system, an amplifier for amplifying speech waves, having an input circuit and an output circuit, a temperature sensitive resistor connected in the input circuit having a. heating element, a circuit for passing a portion of the amplified speech waves in the output circuit through said heater element to control the resistance of said temperature sensitive resistor, the physical dimensions of said resistor and heating element being so small and the heat dissipating factor being so large that the resistance changes of said resistor follow the envelope of speech waves, whereby portions of the speech wave of greatest amplitude are effectively reduced.

3. In combination, an amplifier for signal waves comprising an input circuit and an output circuit, a resistor having a temperature coeflicient of resistance included in the input circuit, a heater element for varying the temperature of said resistor to control the amplitude of signal waves impressed on the amplifier input, a gasfilled space discharge device having a grid or control element, a source of alternating current connected to the discharge terminals of said device in series with said heater element, a circuit coupling the grid of said device to the output circuit of said amplifier, and a source of bias voltage in the grid circuit of said device.

the adjustment of the GAYLON T. FORD. 

